My Computer (right click) -> Properties -> General -> at the
bottom of the Tab, you can see the size of RAM (the actual value
might little different than what is installed 16, 24, 32, 40,
48, 64MB.........)

Systems Information => System Summary => Total Physical Memory

PC Model

In front of the computer (Optiplex GX1 ...)

Systems Information => System Summary => System Model

PC S/N

On the side of PC ( Service Tag: ####### )

Monitor Model

Either in front of the monitor or on the back.

Monitor S/N

On the back of the Monitor.

Circuit Number

Usually written on the jack where the network cable from your
computer plugs into the wall (example: L1345).